1. Field of the Invention
The invention is related to the field of armored electrical cables used for well logging. More specifically, the invention is related to designs for a well logging cable including an optical fiber for signal communication.
2. Description of the Related Art
U.S. Pat. No. 5,495,547 issued to Rafie et al and assigned to the assignee of this invention describes limitations in the signal carrying capacity of electrical well logging cables which have only electrical conductors. The Rafie et al '547 patent discloses a well logging cable having optical fibers which can substitute for one or more of the electrical conductors in a well logging cable to increase the signal carrying capacity.
As is understood by those skilled in the art, well logging cables typically include one or more electrical conductors and armor wires which are wound around the conductors in a pattern designed to maintain a substantially round cross-sectional shape of the cable under repeated applications and relaxations of high axial tension to the cable, while also subjecting the cable to significant bending stresses. Repeated application and relaxation of axial tension and bending stresses occurs mainly as a result of lowering well logging instruments into a wellbore and later removing them from the wellbore by spooling the logging cable through various sheaves which direct the cable into the wellbore. The well logging cables known in the art having only electrical conductors provide good maintenance of the round cross-section of the cable because all of the conductors have similar tensile and bending properties.
Direct substitution of electrical conductors with optical fibers to provide a logging cable having optical fibers can result in the cable having asymmetrical tensile and bending properties, and possibly reduced resistance to deformation of the circular cross-section of the cable. This can be the case even where the optical fibers are enclosed in a steel tube, as disclosed in the Rafie et al '547 patent. Furthermore, some types of well logging cables have only one electrical conductor. The electrical conductor in one-conductor well logging cables is generally straight and has helically wound armor wires surrounding it. The single conductor is straight just as is the center conductor in a multi-conductor well logging cable. The optical fiber/steel tube combination disclosed in the Rafie et al '547 patent is generally not applicable to single conductor logging cables or to being used in the center position in multi-conductor logging cables because of the large axial strain which normally occurs on a "straight" conductor when tension is applied to the cable. For example, a combination fiber-optic/electrical well logging cable having the optical fiber enclosed in a steel tube is disclosed in U.S. Pat. No. 4,522,464 issued to Thompson et al, wherein the optical fiber enclosed in the steel tube is disposed in the center conductor position of a multiple-conductor well logging cable. A drawback to the cable disclosed in the Thompson et al '464 patent is that the steel tube used to enclose the optical fiber is subject to inelastic strain and eventual failure as a result of repeated applications and relaxations of axial tension to the cable. The tube, which is positioned in the center of the cable as disclosed in the Thompson et al '464 patent, is subject to greater axial elongation under tension than the armor wires since the armor wires are helically wound around the axis of the cable whereby elongation of the cable under tension is at least partially dissipated by unwinding of the helical lay of the armor wires as contrasted to the tube which elongates to the same degree as the change in length of the cable under tension.
Excessive axial strain on the tube may also affect another combination fiber optic/electrical well logging cable disclosed in international patent application number WO 94/28450 published under the Patent Cooperation Treaty. The cable disclosed in the WO 94/28450 application also includes an optical fiber enclosed in a metal tube positioned at the center of the cable.
At least one reference suggests using plastic instead of steel for a tube to enclose the optical fiber in a well logging cable. Plastic can have greater strain capacity to a center-located, or a single optical fiber than steel. A plastic tube, without more, is insufficient to provide optical fiber capability to well a logging cable. Another type of combination fiber/optic electrical well logging cable described in "Manufacturing and testing of armored fiber optic downhole logging cable" by Randall et al, Wire Journal, September 1980 provides a plastic-sheathed optical fiber to replace one or more of the electrical conductors in a conventional logging cable. The cable in the Randall et al article has proven commercially unacceptable for well logging because the optical fiber is subject to fluid pressure in the wellbore, since the plastic sheath around the fiber is not pressure sealed. The plastic tubing may be filled with oil in order to exclude wellbore fluids from entering the tube under hydrostatic pressure, but plastic tubes may not provide enough resistance to crushing under high bending stress or high lateral force applied to the cable.